Biamperometric determination of free chlorine, hypochlorite, chlorite and chlorate in sodium chloride brine

Abstract

A method is described for the determination of free chlorine, hypochlorite, chlorite and chlorate in sodium chloride brine originating from an industrial diaphragm electrolysis process. Typical analysis of the electrolysis brine: Cl2 + HOCl/OCl−: 25 mmol/l; ClO2−: <0.2 mmol/l; ClO3−: 25 mmol/l. An essential part of the analysis is the treatment of the sample with an arsenite-phosphate buffer solution (pH 7) in order to prevent loss of free chlorine and undesirable decomposition of hypochlorite into chlorate. Free chlorine and hypochlorite are determined by titrating the excess of arsenite at pH 7–8 with chloramine-T in the presence of iodide. In weakly acidic solution (pH≈0.5)chlorite liberates iodine. The pH is brought back to 8 with hydrogen carbonate; now the iodine reacts with the arsenite and the excess of arsenite is titrated with chloramine-T (Cl2 + ClO− + ClO2−). In a 1.5 M HCl solution and at 100‡C also chlorate reacts with arsenite. Now the excess of arsenite is titrated in acidic solution with chloramine-T in the presence of bromide (total oxidant). The equivalence points are determined biamperometrically with two platinum electrodes (Epol=100 mV).